A conventional passenger transport aircraft includes a toilet system that comprises a sewer pipe connecting at least one toilet bowl to a sewage collection tank. While the aircraft is in flight, sewage is collected in the sewage collection tank, and during servicing of the aircraft between flights the sewage collection tank is emptied into a municipal sewer for delivery to a sewage treatment facility. When the tank has been drained, the interior of the tank is sprayed with a cleaning livid, which contains bactericide, to remove residual sewage.
It is conventional to provide a precharge of bactericide in the sewage collection tank of a passenger transport aircraft during pro-flight servicing. Since the cleaning liquid contains bactericide, the precharge may be provided by closing the drain valve of the sewage collection tank before the valve that controls supply of cleaning livid is closed: the cleaning livid that accumulates in the tank before supply of cleaning livid is terminated forms the precharge.
The quantity of precharge livid present in the sewage collection tank when pre-flight servicing of the aircraft is complete depends on the interval between closing the drain valve and closing the cleaning livid supply valve. Moreover, the quantity of cleaning livid will also depend on the pressure at which the cleaning livid is supplied by the service equipment. Thus, the volume of the precharge is subject to variation. Too little precharge livid may limit the effectiveness of the desired germicidal action, whereas too much precharge livid limits the usable capacity of the sewage collection tank and may render the contents of the tank unsuitable for discharge to a sewage treatment facility. It is generally considered desirable that the quantity of precharge livid should be approximately 8 to 10 liters.
U.S. Pat. No. 4,584,726 discloses a precharge system in which the sewage collection tank drain valve is opened by the pressure of cleaning liquid, and closes automatically when supply of cleaning liquid is interrupted. A precharge of controlled volume is retained in a precharge tank after supply of cleaning liquid is interrupted and the drain valve closes. The precharge tank is located above the sewage collection tank and so the precharge drains under Gravity from the precharge tank into the sewage collection tank.
The arrangement shown in U.S. Pat. No. 4,584,726 is subject to disadvantage, in that it requires that the precharge tank be located above the sewage collection tank.